Houten is not a good place to be if you want to analyse signals in the 100 MHz range …
After looking into some switching regulator artefacts for a while, I’ve concluded that there is nothing wrong with my circuit after all. It’s all due to a nearby FM broadcasting tower!
(367 m high and about 10 km away: the Gerbrandytoren in Lopik – for the Dutch readers)
Getting rid of RF electromagnetic waves is hard. You need a Faraday cage, but you also have to prevent all unwanted RF from entering via measurement and power cables – and it has to work at all the frequencies you’re examining!
I doubt I’ll find a way to get rid of this, but the least I can do is to measure it:
Whoa – nice FM antenna!
Now let’s do the right thing and use a coax cable with internal 50 Ω termination in the scope turned on, and the input sensitivity increased to 1 mV per division:
Again, nice antenna. The “noise floor” is now 20 dB (100x) lower – a much cleaner signal:
And finally, just to make sure it’s really the loop that is picking up the signal, let’s replace that loop by a 50 Ω terminator on the input side as well:
Ah, now we’re getting to the bottom of this – a 30 .. 35 dB reduction:
Note that we’re still picking up very weak FM signals (no cable shielding is perfect). But it’s clear that the incoming signals are coming from antenna-type pick-up of electric fields, not some fault in the measuring setup, or that switcher I was investigating.
Let’s just hope I never have to really chase and analyse signals in this frequency range!